The present invention relates to monitoring patients during surgical procedures in which a general anesthetic has been applied. More specifically, the present invention relates to methods and apparatus for automatically generating an alarm when a patient's depth of anesthesia is insufficient for the surgical procedure being performed.
One important function performed by anesthesiologists relates to maintaining an open airway for a patient throughout a surgical procedure. When a general anesthetic is applied, some type of artificial airway is almost always established for the patient. One popular device for establishing an artificial airway is an endotracheal tube. Another is the well known laryngeal mask airway (LMA).
FIG. 1 shows a perspective view of a prior art LMA 100 and FIG. 2 illustrates an LMA 100 that has been inserted into a patient. LMAs such as LMA 100 are described for example in U.S. Pat. No. 4,509,514. LMA 100 includes an airway tube 110 and a mask portion 130. The mask portion 130 includes a generally elliptical inflatable cuff 134. The tube 110 and mask portion 130 are coupled together and define a continuous, sealed, airway extending from a proximal end 112 of tube 110 to an opening 136 defined in the mask portion 130. LMA 100 also includes an inflation tube 138 for selectively inflating or deflating cuff 134.
In operation, the cuff 134 is initially deflated, and then the mask portion is inserted through the patient's mouth into the patient's pharynx, while the proximal ends of the tube 110 and of the inflation tube 138 remain outside the patient. The mask portion is preferably positioned so that the distal end 140 of cuff 134 rests against the patient's normally closed esophagus and so that the open end 136 is aligned with the entryway of the patient's trachea (i.e., the patient's glottic opening). After the mask portion is so positioned, the cuff is inflated thereby forming a seal around the patient's glottic opening and this establishes a sealed airway extending from the proximal end 112 of the tube 110 to the patient's trachea.
For convenience of exposition, the term “fully inserted configuration” shall be used herein to refer to an LMA that has been inserted into a patient and has the following characteristics: (1) the mask portion is disposed around the patient's glottic opening; (2) the cuff is inflated forming a seal around the patient's glottic opening; (3) the distal tip of the cuff is disposed adjacent the esophageal sphincter; (4) the proximal end of the tube 110 is disposed outside of the patient so that the LMA defines a sealed airway extending from outside the patient's mouth to the patient's lungs (the airway extending from the proximal end 112 of tube 110 to the opening 136 of the mask portion); and (5) the proximal end of the inflation tube remains outside the patient's mouth. FIG. 2 shows an LMA in the fully inserted configuration.
The following steps are normally performed when using an LMA to establish an airway in a patient for use during a surgical procedure. Initially, the patient is anesthetized to a depth that is sufficiently deep for permitting LMA insertion. This process is called induction of anesthesia and is normally accomplished by giving the patient an intravenous injection. The depth of anesthesia required for LMA insertion is less than the depth required for insertion of an endotracheal tube (since the endotracheal tube must pass through the vocal cords, whereas an LMA does not). An LMA is then inserted into the patient and once inserted the anesthesiologist preferably confirms that the LMA has been properly positioned in the fully inserted configuration (e.g., the anesthesiologist preferably confirms that the inflated cuff has formed a seal around the patient's glottic opening). Such checking can be performed by (1) noting whether the patient's chest rises when the anesthesia gas reservoir bag is squeezed and (2) checking for the presence of carbon dioxide in the expired gas and measuring the oxygen saturation. After proper placement of the LMA has been confirmed, the proximal end 112 of the tube 110 may be coupled to a ventilation machine that applies intermittent positive pressure ventilation (IPPV) to the patient and ventilates the patient with a mix of gasses including (1) oxygen; (2) nitrous oxide; and (3) an anesthetic agent. Alternatively, the type of surgery being performed may permit the patient to breathe spontaneously via the LMA during the procedure. After conclusion of the surgical procedure, the LMA is normally removed shortly after the patient becomes responsive to verbal stimulus and can open their mouth when requested to do so.
In addition to establishing and monitoring the patient's airway, another important function performed by anesthesiologists relates to determining the amount of anesthetic drugs to be administered to the patient during a surgical procedure. Briefly, three competing goals for administration of anesthetic drugs during surgical procedures are (1) to induce a level of anesthesia that is “sufficiently deep” so that the patient experiences no pain and remains completely unaware during the surgical procedure; (2) to avoid over-medicating the patient; and (3) to optimize the recovery time. The term “depth of anesthesia” is difficult to define, since it relates to a sleep-like state which is not yet understood in physiological terms. Anesthesiologists nonetheless appreciate when anesthesia is sufficiently deep to meet the needs of preventing reflex response to a surgical incision, since when the level of anesthesia is less than this somewhat elusive threshold the patient will visibly move. Obviously it is undesirable to induce a level of anesthesia that is equal to or less than this threshold and experienced anesthesiologists learn how to keep the majority of their patients sufficiently deeply anesthetized to prevent (1) interference with the course of the procedure due to patient movement and (2) suffering due to the patient regaining consciousness sufficiently to experience pain.
Anesthesia may be General or Local, the former term meaning that unconsciousness is induced, while in the latter a lack of sensation is produced pharmacologically in a specific area of the body. Often, a combination of the two techniques is used, so as to reduce the total amount of general anesthesia required to maintain unconsciousness. This is especially desirable in patients whose condition might be made worse by side-effects of the general anesthetic agents used, for example patients with severe cardiac or pulmonary disease. Often, patients are paralyzed as well as anesthetized. This prevents movement and again reduces the amount of general anesthesia required.
Unfortunately, it is difficult to precisely tailor the drug requirements to maintain a desired level of anesthesia because (1) patients' responses to drugs vary and (2) there is no precise way of measuring anesthetic depth. Hence, patients sometimes recover sufficient consciousness during surgical procedures to experience pain. For example, when seriously ill patients are given minimal amounts of anesthetic drugs to prevent worsening of their underlying condition, and are also paralyzed, it is very difficult to determine their level of awareness.
One way to prevent patients from recalling awareness after a surgical procedure is to administer drugs which suppress short-term memory. Such drugs are highly effective, but when they are given after an episode of awareness has occurred, the actual time during which the patient is aware may cause enormous distress and this is not only undesirable on humanitarian grounds, but may potentially worsen the state of a seriously ill patient.
The practice of anesthesiology is thus a balancing act, in which too much or too little anesthesia may lead to serious or fatal outcomes, quite apart from the risks inherent in the surgical procedure itself. A method of judging anesthetic depth with a degree of precision is therefore highly desirable and it is not surprising that a number of different approaches have been made attempting to solve the problem.
One method relies on observing changes in the patient's cerebral activity as seen on the electro-encephalogram (EEG) while applying auditory stimuli. A more recent method attempts to generate a simple numeric score indicative of the depth of anesthesia by analyzing the patient's complex EEG waveforms. Yet another method detects facial muscle activity and attempts to relate this to pain sensation. Attempts have also been made to relate the activity of the esophageal muscles to depth of anesthesia, but no clear relationship has been found and this method has been abandoned. Some or all of these methods may be combined with information from changes in blood pressure, pulse rate and respiratory rate or depth. An indicator known as the Bispectral Index, or “BIS”, which gives a single number relating to awareness based on EEG analysis, is currently the most popular automatically generated indicator of anesthetic depth.
While these prior art methods may be useful, it would be advantageous to develop other methods and apparatus for estimating a patient's level of anesthesia and in particular for doing so during surgical procedures in which an LMA is used to establish or clear a patient's airway.